The role of angiotensin (Ang) in neurotransmission of calcitonin
gene-related peptide (CGRP)-containing vasodilator nerves in perfused
mesenteric vascular beds isolated from spontaneously hypertensive rats
(SHR) (8- and 15-week-old) and age-matched Wistar Kyoto rats (WKY) was
investigated. In both SHR and WKY preparations precontracted by
continuous perfusion of Krebs' solution containing 7 µM methoxamine
plus 5 µM guanethidine, periarterial nerve stimulation (PNS; 1 and 2 Hz) produced a frequency-dependent vasodilation, which was abolished by
100 nM tetrodotoxin and 500 nM CGRP(8-37) (CGRP receptor antagonist).
The PNS-induced vasodilation in the SHR decreased with age and was
smaller than that in the WKY. The neurogenic vasodilation in the SHR
but not WKY was significantly inhibited by N-acetyltetradecapeptide
renin substrate (RS, 100 and 500 nM), AngI (50 and 100 nM) and AngII
(50 and 100 nM). The inhibitory effects of RS, AngI and AngII were
abolished by the AngII receptor antagonist,
[Sar1,Ile8]AngII (500 nM). The effect of RS
and AngI was inhibited by captopril (5 µM) and temocapril (500 nM).
AngII (100 nM) had no effect on vasodilator response to exogenously
infused CGRP (100 pmol). PNS (2 Hz) of perfused mesenteric vascular
beds increased the release of CGRP-like immunoreactivities (CGRP-LI) in
the perfusate, which was less in 15-week-old SHR than in age-matched
WKY. AngII (100 nM) significantly inhibited the neurogenic release of
CGRP-LI in the SHR but not in the WKY. These results suggest that
exogenous and locally converted AngII, via AngII
receptors, modulates the neurotransmission of CGRP-containing
vasodilator nerves by inhibiting CGRP release from the nerve.
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Introduction |
The
tone of resistance blood vessels is maintained mainly by sympathetic,
adrenergic vasoconstrictor nerves, but recent studies have revealed the
NANC vasodilator innervation in blood vessels and involvement of
vasodilator nerves in the regulation of vascular tone (Kawasaki
et al., 1988
; Toda and Okamura, 1992). We have demonstrated
that the mesenteric resistance blood vessels of the rat are innervated
by NANC vasodilator nerves in which CGRP, a potent vasodilator
neuropeptide, acts as the vasodilator neurotransmitter (Kawasaki
et al., 1988, 1990a, 1990b, 1991). In relation to control of
the vascular tone, CGRP-containing vasodilator nerves (CGRP nerves)
suppress vasoconstrictor responses to adrenergic nerve stimulation
through released CGRP, and conversely, adrenergic nerves inhibit the
release of CGRP from the nerve to decrease CGRP nerve function
(Kawasaki et al., 1990a,b, 1991). Thus, we have proposed
that CGRP vasodilator nerves along with sympathetic vasoconstrictor
nerves regulate the tone of the mesenteric resistance blood vessels.
Evidence has accumulated showing that increased total peripheral
vascular resistance maintains elevated blood pressure in chronic
hypertension (Folkow et al., 1970; Zimmerman, 1983). The impaired function of the control systems regulating peripheral resistance has been considered (Zimmerman, 1983; Head, 1989). In
studies with SHRs, which are the best animal models for human essential
hypertension, the enhanced activity of sympathetic vasoconstrictor nerves has been an important factor in the increased tone of peripheral resistance vessels (Head, 1989; Kawasaki et al., 1982b).
Recently, we demonstrated that the CGRP nerve function in SHR decreases with age and proposed that malfunction of CGRP vasodilator nerves regulating peripheral vascular resistance plays an important role in
the development and maintenance of hypertension in SHR (Kawasaki et al., 1990c; Kawasaki and Takasaki, 1992). However, the
mechanisms underlying reduced function of CGRP vasodilator nerves in
SHR remain unresolved. In a recent pharmacological study, we showed that chronic treatment of SHR with an ACE inhibitor, but not other antihypertensive drugs such as a calcium antagonist and beta
adrenoceptor antagonist, prevents the decrease in vasodilator responses
mediated by CGRP nerves (Kawasaki, 1992). This finding suggested that
the renin-angiotensin system might be involved in the reduced function of CGRP vasodilator nerves in SHR. The present study was undertaken to
investigate the role of Ang in neurotransmission of CGRP vasodilator nerves in SHR. We report here that the locally generated AngII in the
blood vessel wall of the SHR caused an inhibition of neurotransmissions of CGRP nerves.
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Materials and Methods |
Blood pressure measurement.
Male SHRs at 8 and 15 weeks of
age and age-matched normotensive WHYs, purchased from Charles River
Japan (Shizuoka, Japan), were anesthetized with an intraperitoneal
injection of sodium pentobarbital (50 mg/kg). The left carotid artery
was cannulated, and the arterial pressure was measured with a pressure
transducer (model P23ID) and recorded on a polygraph (model RM-6000,
Nihon Kohden).
Perfusion of the mesenteric vascular bed.
Under
pentobarbital anesthesia, the mesenteric vascular bed was isolated and
prepared for perfusion as described previously (Kawasaki et
al., 1988, 1990a). The superior mesenteric artery was cannulated
and gently flushed with a modified (see below) Krebs-Ringer bicarbonate
solution (Krebs' solution) to eliminate blood from the vascular bed.
After removal of the entire intestine and associated vascular bed, the
mesenteric vascular bed was separated from the intestine by cutting
close to the intestinal wall. Only four main arterial branches from the
superior mesenteric trunk running to the terminal ileum were perfused.
All other branches of the superior mesenteric artery were tied off. The
preparation was perfused with Krebs' solution at a constant flow rate
of 5 ml/min with a peristaltic pump (model SJ-1215, ATTO Co., Tokyo, Japan). The preparation was also superfused with the same solution at a
rate of .5 ml/min to prevent drying. Modified Krebs' solution with the
following composition was used (mM): NaCl, 120.0; KCl, 5.0;
CaCl2, 2.4; MgSO4, 1.2;
NaHCO3, 25.0; 2NaEDTA, 0.027; and dextrose, 11.0 (pH 7.4). The Krebs' solution was bubbled with a mixture of 95%
O2-5% CO2 before passage
through a warming coil maintained at 37°C. Changes in the perfusion
pressure were measured with a pressure transducer (model MPU-0.5A).
PNS and bolus injection of CGRP.
After allowing the basal
perfusion pressure to stabilize, the preparation was contracted with
methoxamine at a submaximal concentration of 7 µM in the presence of
5 µM guanethidine, which was added to block adrenergic
neurotransmission. The increased perfusion pressure was allowed to
stabilize, and the preparation was subjected to PNS at 1 and 2 Hz or to
bolus infusion of CGRP. PNS was applied for 30 sec through bipolar
platinum ring electrodes placed around the superior mesenteric artery.
Rectangular pulses of 1 msec in duration and supramaximum voltage (50 V) were applied by an electronic stimulator (model SEN 3301, Nihon
Kohden). In another preparation, 100 pmol CGRP diluted with Krebs'
solution containing methoxamine and guanethidine was infused directly
into the perfusate proximal to the arterial cannula with an infusion pump (model 975, Harvard Apparatus, S. Natick, MA). The volume of
infusion was 100 µl for 10 sec. After control responses to the first
PNS at 2 Hz (S1) and the first injection of CGRP
(I1) were obtained, the second PNS
(S2) and the second infusion of CGRP
(I2) were carried out during perfusion with the
final concentrations of AngI, AngII and RS. After switching Krebs'
solution containing methoxamine plus guanethidine without the test
drug, the third PNS (S3) and the third infusion
(I3) were carried out as the after-control.
In separate experiments, the Krebs' solution containing methoxamine,
guanethidine and the final concentration of AngII receptor antagonist
([Sar1, Ile8]AngII) or
ACE inhibitor (captopril or temocapril) was perfused throughout the
experiment.
To estimate the effects of the drugs tested, the changes in perfusion
pressure in response to PNS or bolus infusion of CGRP were expressed as
the ratio between the vasodilation induced by S2
and by S1, or the vasodilation induced by
I2 and by I1, respectively. At the end of each experiment, 100 µM papaverine was perfused through
the preparation to produce complete relaxation. Vasodilation was
expressed as a percentage of the maximum relaxation induced by
papaverine.
Release of CGRP-like immunoreactivity.
The mesenteric
vascular beds isolated from 15-week-old SHRs and age-matched WKYs were
perfused with Krebs' solution containing 7 µM methoxamine and 5 µM
guanethidine at a constant flow rate of 5 ml/min and superfused with
Krebs' solution (0.5 ml/min). The perfusate was collected for 5 min
before and after PNS (S1 and
S2) at 2 Hz for 30 sec. The drugs tested were
perfused 10 min before and throughout PNS (S2).
Each sample was applied to a Sep-PakC18 cartridge
(Waters Associates, Milford, MA), and the absorbed peptide was eluted
with 3 ml of 60% acetonitrile in 0.1% trifluoroacetic acid. The
eluate was evaporated under a vacuum and stored at 
80°C until a
radioimmunoassay for CGRP as described previously (Kawasaki et
al., 1990c; Kawasaki and Takasaki, 1992). The samples were
preincubated with rabbit anti-human CGRP-II serum (Peninsula
Laboratories, Inc., Melmont, CA) at 4°C for 12 hr. Then, the reaction
mixture was incubated with
(2-[125I]iodohistidyl10)CGRP
(human) (Amersham International, Buckinghamshire, UK) for an additional
24 to 36 hr at 4°C. The antibody-bound antigen was separated from
free antigens by the double-antibody preincubation method. The
antibodies used cross-reacted 100% with rat and human CGRP but 0%
with substance P, neuropeptide Y and AngII. The lower detection limit
was 1 fmol/tube for CGRP-LI.
Statistical analysis.
All data are presented as the
mean ± S.E.M. One-way analysis of variance followed by Dunnett's
test or by Tukey's test was used to determine the significance between
values of different ages or different doses. Unpaired Student's
t test was used to determine the significance of differences
between two means. A value of P < .05 was considered significant.
Drugs.
The following drugs were used: AngI (Sigma Chemical
Co., St. Louis, MO), AngII (Sigma), N-acetyltetradecapeptide (Sigma), captopril (Sankyo Pharmaceutica, Tokyo, Japan), guanethidine sulfate (Tokyo Kasei, Tokyo, Japan), human CGRP(8-37) (Peptide Institute, Osaka, Japan), methoxamine HCl (Nihon Shinyaku, Kyoto, Japan), rat
-CGRP (Peptide Institute), [Sar1,
Ile8]AngII (Sigma), temocapril diacid (active
form of temocapril, Sankyo) and TTX (Sigma). All drugs, except for
temocapril diacid which was dissolved in 0.001%
NaHCO3, were dissolved in distilled water and
diluted with Krebs' solution containing 7 µM methoxamine and 5 µM
guanethidine, when injected as a bolus or perfused.
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Results |
Blood pressure, perfusion pressure and vasoconstriction induced by
methoxamine in SHRs and WKYs.
As shown in table
1, the mean carotid arterial pressure in
SHRs at 15 weeks of age was significantly greater than that at 8 weeks
of age; no significant difference was found in the WKY blood pressure
at any age. The mean carotid arterial pressures in SHRs at 8 and 15 weeks of age were significantly greater than those of the age-matched
WKYs. The resting mean perfusion pressure of perfused mesenteric
vascular beds was significantly greater in the 15-week-old SHRs than
the 15-week-old WKYs.
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TABLE 1
Mean blood pressure, mean perfusion pressure, methoxamine-induced
vasoconstriction and vasodilation of the mesenteric artery induced by
PNS in normotensive WKYs and SHRsa
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Neurogenic vasodilator response in SHRs and WKYs.
To maintain
the active tone of the mesenteric artery, the preparation was
contracted by the continuous perfusion of 7 µM methoxamine in the
presence of 5 µM guanethidine, which was added to block adrenergic
neurotransmission. As shown in table 1, the methoxamine-induced increases in mean perfusion pressure before PNS were significantly greater in the SHR than in the WKY.
As shown in figure 1A, PNS at 1 and 2 Hz
in the SHR preparations with active tone produced a frequency-dependent
decrease in perfusion pressure because of vasodilation, which was
abolished by perfusion of TTX (100 nM). The bolus infusion of CGRP (100 pmol) also caused a long-lasting vasodilation, which mimicked the
response to PNS (fig. 1B). The perfusion of CGRP(8-37) (500 nM), a
CGRP receptor antagonist, markedly inhibited the vasodilator response
induced by both exogenous CGRP and PNS (fig. 1B), which indicates that
the vasodilator response to PNS is neurogenic and mediated by CGRP
vasodilator nerves. Similar results were obtained in the WKY
preparations.
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Fig. 1.
Typical records showing vasodilator responses to
PNS ( ) and bolus infusion of CGRP ( ) and the effects of perfusion
of TTX (100 nM) (A) or CGRP receptor antagonist, CGRP(8-37) (500 nM) (B), in the perfused mesenteric vascular beds from 15-week-old SHRs.
The vascular tone was increased by perfusion of methoxamine (7 µM) in
the presence of guanethidine (5 µM). PPV, papaverine perfusion.
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The PNS-induced (1 and 2 Hz) vasodilator responses in preparations from
8-week-old SHRs were similar in magnitude to the vasodilation in
age-matched WKY preparations, but the PNS-induced vasodilation in
15-week-old SHRs was significantly less than that in 8-week-old SHRs
and 15-week-old WKYs (table 1). Only a small reduction in the
PNS-induced vasodilation occurred with age in the WKY preparations (table 1).
Effects of AngI and AngII on neurogenic vasodilator response.
As shown in figure 2 and table
2, the perfusion of AngI (50 and 100 nM)
and AngII (50 and 100 nM) in both WKY and SHR preparations caused a
transient increase in perfusion pressure because of vasoconstriction in
a concentration-dependent manner. The vasoconstrictor responses to AngI
and AngII were significantly greater in the SHR preparations than in
the WKY preparations (table 2). No significant difference in the AngI-
and AngII-induced vasoconstrictions between 8-week-old SHRs and
15-week-old SHRs or between 8-week-old WKYs and 15-week-old WKYs
occurred, however (table 2).
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Fig. 2.
Typical records showing the effects of AII (100 nM)
on vasodilator responses to PNS (2 Hz) in the absence (B) and presence (C) of AII receptor antagonist,
[Sar1,Ile8]AII (500 nM), in the perfused
mesenteric vascular bed from 15-week-old spontaneously hypertensive
rats (SHR). S1, S2 and S3 indicate the first, second and third PNS, respectively. AII was perfused throughout S2. The vascular tone was raised by perfusion of
methoxamine (7 µM) in the presence of guanethidine (5 µM). A,
control responses. PPV, papaverine perfusion.
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TABLE 2
Vasoconstrictor responses to perfusion of AngI and AngII and RS in the
perfused mesenteric vascular bed with active tone produced by
methoxamine (5 µM) and guanethidine (7 µM) in WKYs and
SHRsa
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In preparations from 8- and 15-week-old SHRs, both AngI (50 and 100 nM)
and AngII (50 and 100 nM) caused a concentration-dependent inhibition
of the vasodilator response to PNS (figs. 2 and
3). The inhibitory effect of AngI and
AngII was much greater in 15-week-old SHRs than in 8-week-old SHRs.
However, in WKY preparations from both ages, neither AngI nor AngII
inhibited the PNS-induced vasodilation (fig. 3).
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Fig. 3.
Effects of angiotensin I (AI) and II (AII) on
vasodilator responses to periarterial nerve stimulation (2 Hz) in
perfused mesenteric vascular beds from 8- and 15-week-old (wks) WKYs
(circles) and SHRs (triangles).   , control ratio without
angiotensins; , ratio in the presence of AI or AII
concentrations of 50 and 100 nM. *P < .01, **P < .01, compared with control (Dunnett's test).
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Effects of AngII receptor antagonist and ACE inhibitors on AngI-
and AngII-induced inhibition.
In both 15-week-old SHRs and WKYs,
vasoconstrictor responses to AngI and AngII were abolished by the AngII
antagonist,
[Sar1,Ile8]AngII (500 nM), and the AngI-induced vasoconstriction was inhibited by the ACE
inhibitors captopril (5 µM) and temocapril (500 nM) (table
3).
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TABLE 3
Effects of AngII receptor antagonist
([Sar1,Ile8]AngII, 1 µM) and AngI-converting enzyme
inhibitors (captopril, 5 µM; temocapril, 500 nM) on vasoconstrictor
responses to AngI and AngII and RS in the perfused mesenteric vascular
bed with active tone produced by methoxamine (5 µM) and guanethidine
(7 µM) in WKYs and SHRsa
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In preparations from 15-week-old SHRs, the inhibitory effects of AngI
and AngII on the neurogenic vasodilation were antagonized by the AngII
antagonist,
[Sar1,Ile8]AngII (500 nM)
(fig. 4), and the effect of AngI was also
inhibited by captopril (5 µM) and temocapril (500 nM). In contrast,
in the preparations from WKYs, the neurogenic vasodilation in response to PNS was not affected by the AngII antagonist or ACE inhibitors.
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Fig. 4.
Effects of the angiotensin (A) receptor antagonist,
[Sar1,Ile8]AII (500 nM), and AngI-converting
enzyme inhibitors, captopril (Capt, 5 µM) and temocapril (Temcp, 500 nM) on the inhibitory effect of AngI (A I) and AngII (A II) in the
perfused mesenteric vascular beds from 15-week-old (wks) WKYs ( ) and
SHRs ( ). *P < .05,**P < .01, compared with AngI and
AngII alone (Dunnett's test).
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Effect of AngI and AngII on CGRP-LI release induced by PNS.
In
the preparations from 15-week-old SHRs and WKYs, PNS at 2 Hz evoked an
increase in CGRP-LI release in the perfusate, which was significantly
less in the SHRs than in the WKYs (WKY, 0.8373 ± 0.114 fmol/ml,
n = 14; SHR, 0.2877 ± 0.042 fmol/ml,
n = 15; P < .01, Student's t test).
This release was abolished by TTX (100 nM) and
Ca++ removal from the medium (data not shown). As
shown in table 4, AngI and AngII (100 nM)
in SHR but not WKY preparations inhibited the release of CGRP induced
by PNS. A significant difference from the control was found in the
inhibition of AngII.
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TABLE 4
Effect of AngII on release of CGRP-LI (fmol/ml) induced by PNS in
perfused mesenteric vascular beds with active tone in SHRs and
WKYsa
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Effect of AngII on vasodilator response to bolus infusion of
exogenous CGRP.
In preparations from 15-week-old SHRs and WKYs, a
bolus infusion of exogenous CGRP (100 pmol) caused a long-lasting
reduction in perfusion pressure because of vasodilation, which was
antagonized by CGRP(8-37) (fig. 1B). The control
I2/I1 ratios in SHR and WKY preparations were 0.974 ± 0.062 (n = 6) and
1.002 ± 0.025 (n = 6), respectively. The
perfusion of AngII (100 nM) did not affect the vasodilator response to
bolus infusion of exogenous CGRP: I2/I1 ratio in SHR
(n = 6) and WKY (n = 6) preparations,
1.0071 ± 0.070 and 0.967 ± 0.051, respectively.
Effect of RS on neurogenic vasodilation induced by PNS.
As
shown in figure 5 and table 2, the
perfusion of RS (100 and 500 nM) in WKY and SHR preparations caused a
transient increase in perfusion pressure, because of vasoconstriction,
in a concentration-dependent manner. The vasoconstrictor responses to
RS were significantly greater in SHR than in WKY (table 2), but no
significant difference in the RS-induced vasoconstriction between
8-week-old SHRs and 15-week-old SHRs or between 8-week-old WKYs and
15-week-old WKYs occurred (table 2). In preparations from 15-week-old
SHRs and WKYs, the vasoconstrictor response to 500 nM RS was abolished by [Sar1,Ile8]AngII (500 nM) and inhibited by ACE inhibitors (5 µM captopril and 500 nM
temocapril), more so in WKYs than in SHRs (table 3).
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Fig. 5.
Typical records showing the effect of RS (500 nM)
on vasodilator responses to PNS (, 2 Hz) in the absence (B) and
presence (C) of the AngII (A II) receptor antagonist,
[Sar1,Ile8]AII (1 µM), in the perfused
mesenteric vascular bed from 15-week-old SHRs. S1,
S2 and S3 indicate the first, second and third PNS, respectively. RS was perfused throughout S2. The vascular
tone was increased by continuous perfusion of methoxamine (7 µM) in the presence of guanethidine (5 µM). A, control responses; PPV, papaverine perfusion.
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In preparations from 15- but not 8-week-old SHRs, RS (100 and 500 nM)
caused a significant inhibition of the vasodilator response to PNS
(figs. 5 and 6). The inhibitory effect of
RS was much greater in the 15-week-old SHRs than in the 8-week-old
SHRs. In WKY preparations of both ages, however, RS did not inhibit the
PNS-induced vasodilation (fig. 6).
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Fig. 6.
Effect of RS on vasodilator responses to PNS (2 Hz)
in perfused mesenteric vascular beds from 8- and 15-week-old (wks) WKYs (circles) and SHRs (triangles). , control ratio without RS; , ratio in the presence of RS at concentrations of 100 and 500 nM. **P < .01, compared with control (Dunnett's test).
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In 15-week-old SHRs but not in WKYs, the inhibitory effect of RS on
PNS-induced vasodilation was antagonized by the AngII antagonist and
ACE inhibitors (fig. 7).
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Fig. 7.
Effects of the AngII (A II) receptor antagonist,
[Sar1,Ile8]AII (500 nM), and ACE inhibitors,
captopril (Capt, 5 µM) and temocapril (Temcp, 500 nM) on the
inhibitory effect of RS in the perfused mesenteric vascular beds from
15-week-old (wks) WKYs () and SHRs (). *P < .05, **P < .01, compared with RS alone (Dunnett's test).
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Discussion |
The present study demonstrated that frequency-dependent
vasodilator responses to PNS in the mesenteric arteries isolated from both SHRs and WKYs were abolished by the neurotoxin TTX and the CGRP
receptor antagonist CGRP(8-37), which indicates that the responses are
neurogenic and mediated by endogenous CGRP which is released by CGRP
nerves. These results agree with previous reports (Kawasaki et
al., 1990c), and confirm the earlier findings that neurogenic
vasodilation in response to CGRP nerve stimulation in aged SHRs is less
than in age-matched WKYs and decreases with age, and that the
neurogenic release of CGRP-LI evoked by PNS in aged SHRs was
significantly less than that in age-matched WKYs (Kawasaki et
al., 1990c; Kawasaki and Takasaki, 1992). Thus, the present
results support our proposal that the function of CGRP nerves in SHRs
decreases with age.
In the present study, both AngI and AngII inhibited the neurogenic
vasodilator response to PNS in SHRs but not in WKYs. The inhibitory
effect of AngI and AngII was abolished by the AngII receptor
antagonist,
[Sar1,Ile8]AngII, which
indicates that the effect is mediated by AngII receptors. The effect of
AngI was also inhibited by the ACE inhibitors captopril and temocapril,
which suggests that AngII locally converted from AngI in the mesenteric
vasculature is involved in this effect in vitro.
Additionally, RS attenuated the neurogenic vasodilation in response to
the PNS of the SHR preparations, which was antagonized by the AngII
receptor antagonist and by ACE inhibitors. Thus, AngII locally
converted in the vasculature is responsible for the inhibitory effect
of RS.
Evidence for the presence of a local renin-angiotensin system in the
blood vessel wall is compelling (Mizuno et al., 1988; Dzau,
1989; Ziogas and Story, 1991; Hilgers et al., 1991). Such a
local system in the vasculature could synthesize and release AngII,
which exerts local autocrine-paracrine influences on the vascular
function (Malik and Nasjletti, 1976; Kawasaki et al., 1984).
This concept supports the present suggestion that AngI and RS act
through AngII, which is locally converted in the mesenteric vasculature.
The results of the present study show that AngII caused a decrease in
the neurogenic release of CGRP-LI in response to PNS of the SHR
preparations but not the WKY preparations. However, AngII had no
significant effect on vasodilator responses to exogenously applied CGRP
in the SHR preparations. These results strongly suggest that AngII acts
on a presynaptic site of CGRP nerves to decrease the neurogenic release
of CGRP. It is widely accepted that AngII increases the release of
neurotransmitter to enhance the autonomic neurotransmission, especially
in the vasculature. However, several reports have shown that AngII has
the capacity to inhibit the autonomic neurotransmission by decreasing
the release of transmitter in the rabbit ear artery (Ronai, 1990), pig
renal artery (Ferguson and Randall, 1989) and rabbit vas deferens
(Trachte, 1988). Additionally, AngII has been reported to inhibit the
release of acetylcholine in the human and rat cerebral cortex (Barnes
et al., 1989, 1990). The release of endogenous dopamine in
the pig renal artery seemed likely to play a part in this inhibition
(Ferguson and Randall, 1989). In the rabbit ear artery (Ronai, 1990), a
PG-mediated mechanism has been proposed because AngII stimulates the
synthesis of PGI2 and PGE2
(Dusting et al., 1981). However, in the rabbit vas deferens, PGs did not mediate the inhibitory effect of AngII on the autonomic neurotransmission (Trachte, 1988). Furthermore, PGs such as
PGE1 but not PGI2 have been
reported to enhance the CGRP release from the capsaicin-sensitive
nerves (Franco-Cereceda, 1989). Taken together, these findings suggest
that the PG-mediated mechanism is unlikely to play a major role in the
inhibitory effect of AngII on the neurotransmission of CGRP nerves. It
is conceivable that AngII acts directly on a presynaptic site of action
(probably AngII receptors) in the CGRP nerve to inhibit the release of
CGRP.
The present results also show that the inhibitory effect of AngII on
the neurotransmission of CGRP nerves was specific to the SHR
preparations and became clear with age. This finding raises the
hypothesis that SHRs have an altered function and sensitivity to AngII
receptors, which are probably present on the CGRP nerves in the
resistance blood vessel; or the CGRP nerves of SHRs become newly
endowed with such presynaptic AngII receptors as the SHRs age. Such an
altered sensitivity to AngII receptors in the adrenergic neurotransmission has been shown by the finding that the facilitator effect of AngII is enhanced in the mesenteric vasculature of SHRs (Kawasaki et al., 1982a).
Previous studies demonstrated that the CGRP nerves inhibit the
adrenergic nerve-mediated vasoconstriction via direct
vasodilation of CGRP released (Kawasaki et al., 1990a),
whereas the adrenergic nerves decrease the release of CGRP from the
CGRP nerves to inhibit CGRP nerve-mediated function (Kawasaki et
al., 1990b, 1991). Moreover, we have observed that the CGRP
nerve-mediated function in the SHR mesenteric vasculature decreases
with age (Kawasaki et al., 1990c; Kawasaki and Takasaki,
1992). The current findings that RS, AngI and AngII attenuate the
neurotransmission of CGRP nerves in the SHRs suggest that the vascular
AngII, which is locally converted in the resistance blood vessel,
contributes to the decreased function of CGRP nerves. This notion is
supported by the previous finding that long-term treatment of SHRs with
the ACE inhibitor prevents the decreased vasodilation mediated by CGRP
nerves (Kawasaki, 1992).
Although increased levels of renin, which is synthesized in the blood
vessel wall and partly taken up from the plasma (Inagami et
al., 1991; Mizuno et al., 1986), have been reported in
SHR blood vessels (Naruse and Inagami, 1982), recent studies have presented little evidence for increased production of AngII in the
blood vessels of SHRs (Nakamura et al., 1986). On the
contrary, the decreased mRNA level of angiotensinogen in periarterial
adipocytes taken from SHRs suggested the reduced production of AngII
(Naftilan et al., 1988). However, there is evidence that
sensitivity to AngII receptors can be enhanced in the mesenteric artery
of the SHR (Kawasaki et al., 1982a, 1984; Li and Jackson,
1989). In fact, the present experiments showed that the SHR
vasoconstrictor responses to perfusion of AngI and AngII were
significantly greater than those in WKY preparations. In addition to
this mechanism, the current finding that AngII has the ability to
inhibit the function of vasodilator nerves only in SHRs suggests that
the vascular renin-angiotensin system could induce increased vascular
tone without an elevated concentration of AngII in the blood vessel wall.
In conclusion, the exogenous and locally converted AngII in the SHR
mesenteric artery has the ability to inhibit the neurotransmission of
CGRP vasodilator nerves. The present results suggest that the reduced
function of CGRP nerves by the vascular renin-angiotensin system plays
an important role in the development and maintenance of chronic
hypertension.
ACE, angiotensin converting enzyme;
Ang, angiotensin;
CGRP, calcitonin gene-related peptide;
CGRP-LI, calcitonin
gene-related peptide-like immunoreactivities;
NANC, nonadrenergic
noncholinergic;
PG, prostaglandin;
PNS, periarterial nerve stimulation;
RS, N-acetyltetradecapeptide renin substrate;
SHR, spontaneously
hypertensive rat;
TTX, tetrodotoxin;
WKY, Wistar Kyoto rat.
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Abstract
Introduction
-adrenergic stimulation in rat.
J Cardiovasc Pharmacol 8:suppl.10, S1-S5. 
